This disclosure pertains to the operation and maintenance of chemical plants and refineries. More specifically, the present disclosure relates to the process for cleaning the internal surfaces of chemically contaminated reactors, packed beds, absorbent chambers, compressors, pipes, connectors and other equipment.
Refineries and chemical plants must perform turnarounds on chemical processing units which utilize reactors and other vessels containing packed media. The purpose of these turnarounds is to replace catalysts or other media that have lost the ability to perform. Performance measures include catalyst activity, pressure drop, yields, molecular sieve selectivity, etc.
When the turnarounds are being performed, the facility cannot upgrade refined products to higher value streams, resulting in irreversible loss of revenue to the refinery or chemical plant. Therefore, an incentive exists to minimize the duration of the outage and perform the change-out of the media as quickly and effectively as possible, while maintaining a safe work environment.
Moreover, new developments in environmental regulations and enforcement have led to more stringent emissions requirements. One of the major developments resulting from these regulations is the desire to minimize flaring from refining equipment. Many facilities have installed Flare Gas Recovery Units (FGRUs) to capture gases in the flare system and return them to the fuel gas system rather than flaring continuously. FGRUs typically consist of one or more liquid ring compressors capable of taking low pressure flare gas and pushing it into the fuel gas system or other medium pressure system. These new units are often mandated by Consent Decree agreements between refiners and the Environmental Protection Agency (EPA). As a result, there is significant environmental incentive to avoid flaring and to keep the gases within the constraints of the FGRUs when gases must be vented from the equipment. These constraints may include, for example, the following parameters.
1) Flow Rate:
The compressors are designed to capture a limited quantity of vapors in the flare system. If the compressors are overwhelmed the gas will be flared.
2) BTU Value:
Nitrogen is frequently used to clear noxious chemicals from refining equipment. There is a limitation on how much nitrogen can be sent to the fuel gas system via the FGRU because the nitrogen, which has no heating value, dilutes the fuel gas system and causes the plant heaters to operate abnormally. This can lead to further upsets, so the plant fuel gas BTU value is closely monitored.
3) Temperature:
Because the compressors are liquid ring compressors, there is a temperature limit which protects the compressors. Generally, temperatures above 170° F. are not allowed.
The process vessels are generally at the heart of a hydrocarbon processing facility but often cannot be isolated from adjacent supporting equipment. For example, a typical hydrotreating process unit in a petroleum refinery has a reactor containing a metal catalyst, a hydrogen compressor, shell and tube heat exchangers, a heater, air cooled fin tube exchangers, piping and other miscellaneous pressure vessels. All equipment in the process circuit can be collectively referred to as the reactor circuit. When a turnaround occurs on such a unit, the entire reactor circuit must be cleaned together because the compressor and heat exchangers are used to circulate a gas used to cool down the reactor at a regulated rate.
Under most circumstances, it may be desirable to ensure that the equipment in a reactor circuit are not exposed to water or steam due to concerns about technical items such as metallurgy, loss of catalyst activity and the destruction of expensive absorbent materials such as molecular sieves. Additionally, there are practical concerns with respect to materials inside the equipment which may form clumps when soaked with water, making them difficult to remove. Moreover, in the case of reactors in hydrotreating units, the shutdown and cool down procedure requires that the hydrogen compressor in the system remain online, and because hydrogen compressors cannot pump steam, it must be cleaned without using steam or aqueous cleaners that are otherwise commonly used in the industry.
One previously disclosed method for preparing reactor circuits for safe work involves a “hot sweep,” where the heater in the reactor loop is used to raise the hydrogen stream temperature levels high enough to strip the heavy hydrocarbons from the catalyst as the hydrogen compressor circulates the gas. Following that step, the hydrogen is replaced with nitrogen by repetitively depressurizing the system to the flare system and pressuring it back up with nitrogen (commonly called a “huff and puff”). At that point, the compressor is restarted, sending the nitrogen through the reactor circuit at the same time that the continuous injection and purge of nitrogen is occurring. The purge stream is sent to the flare system. The process gradually decreases the concentration of noxious gases in the circuit and cools down the reactor. Depending on the design of the compressor, nitrogen availability and other considerations, the operator may use other gases instead of nitrogen, including purchased fuel gas (ethane and methane). These processes require enormous quantities of nitrogen, which is costly. The goal of the entire operation is to render the circuit safe for work (0% LEL, 0 PPM H2S and <100° F.). Depending on the size and state of the unit, the entire effort can take 3 or more days.
In cases where the “huff and puff” and nitrogen purge steps are sent to a flare system with an FGRU, the constraints mentioned above will govern the flow rate and therefore will set the duration of the activity. In systems that include flare gas recovery, the FGRU becomes the limiting factor of all or most hydrotreater shutdowns.
Another method known in the field for safely removing contaminated catalyst from a reactor is to perform a “wet dump.” After the equipment is cooled down, the reactor is filled with water. The catalyst is subsequently dumped wet, effectively preventing fires and other hazards. Challenges to this method are time (system must be cooled down prior to introducing water), safe handling and disposal of hot water, increased amount of waste for disposal and difficulties involved in controlling a large system filled with hot catalyst and metal, mixed with cool water.
Although it is possible in some cases to isolate a process vessel for cleaning and decontamination, it is not always practicable to use steam or aqueous solutions to clean the equipment. For instance, a compressor is typically not available for circulating gas through the process internals. One such example is an adsorbent chamber in the Parex™ Process (UOP technology). One method for removing noxious gases from such equipment is purging with an inert gas, most commonly nitrogen. A common method is to pressure a system with nitrogen up to a certain pressure, then vent it down to a low pressure. These steps may be repeated until the atmosphere inside the system meets environmental and safety limits.
In some cases, a continuous flow of nitrogen is introduced at one point in a system while the same amount is vented (either to the flare system or to the atmosphere) at another point. The nitrogen reduces contaminants in the vessel through dilution. Often the equipment is vented to the flare during the nitrogen purges; however, purging directly to the atmosphere is possible once environmental limits have been reached. At that point, the vessel is opened at several points to the atmosphere and air blowers are used to remove the nitrogen and the last traces of noxious gases. The end goal of all of the processes involving nitrogen or other gases is to render the equipment dry of free oil and the internal atmosphere free of noxious gases.
In summary, most of these known methods are time-consuming and/or expensive to implement. Furthermore, any solution that requires further cleaning inside a confined space may introduce safety risk to the workers implementing the process.